Inductive coupling between a pair of coils is a very promising technology for wireless power and data transmission to implantation components in many biomedical applications. For a retinal prosthesis specifically, a receiving coil with high self-inductance and low series resistance (i.e., high Q) is needed to optimize the efficiency of the system. However, thin film coils made by existing conventional planar micromachining technology cannot achieve this requirement due to geometrical restrictions. Some techniques, such as electroplating and suspended structures (see, for example, Jae Y. Park and Mark G. Allen, High Q spiral-type microinductors on silicon substrates, IEEE Transactions on Magnetics, Vol. 35, NO. 5, (1999) 3544-3546), have been developed to make high-Q coils, but the processes are usually expensive, complicate, and unreliable.
FIG. 1 shows the concept of a telemetry system used for biomedical applications by way of an equivalent circuit of an electromagnetically coupled system. The primary coil (L1) is outside the human body, and thus has fewer design constraints such as physical sizes and power consumption. Therefore, the power transfer efficiency of this system mainly depends on the intrinsic characteristics of the receiving coil (L2). The intrinsic characteristics of a planar circular coil, i.e., the self-inductance Ls and series resistance Rs, can be calculated by its geometrical factors, as shown in equation (1) and (2).
                                          L            s                    =                      2            ⁢            π            ⁢                                                  ⁢                          dN              2                        ×                                          10                                  -                  9                                            [                                                                    (                                          ln                      ⁢                                                                        4                          ⁢                          d                                                t                                                              )                                    ⁢                                      (                                          1                      +                                                                                                    t                            2                                                                                24                            ⁢                                                          d                              2                                                                                                      ⁢                        …                                                              ⁢                                                                                  )                                                  -                                  1                  2                                +                                                                            43                      ⁢                                                                                          ⁢                                              t                        2                                                                                    288                      ⁢                                                                                          ⁢                                              d                        2                                                                              ⁢                  …                                            ⁢                                                          ]                        ⁢                                                  ⁢                          (              Henries              )                                      ,                            (        1        )                                          R          s                =                  ρ          ⁢                      L                          A              c                                ⁢                                          ⁢                      (            Ohm            )                                              (        2        )            
Where Ls is the self-inductance, N is the number of turns, d (in cm) is the mean diameter of the coil, t (in cm) is the coil width, Rs is the series resistance, ρ is the metal resistivity, L is the total wire length, and Ac is the cross section area of the metal wire. See, for example, Herbert Dwight, Electrical Coils and Conductors, McGraw Hill Book Company, 1945, ch 31, p 267. With these known parameters, the intrinsic Q-factor, which represents the efficiency of an inductor, is defined by the following formula, where ω is the angular resonant frequency of the AC signal (i.e., 2π×1 MHz for the current system),
                              Q          t                =                              ω            ⁢                                                  ⁢                          L              s                                            R            s                                              (        3        )            
Theoretically, the higher the Q-factor, the more efficiently the coil performs. That means the power transfer efficiency of the system can be improved significantly by both increasing the self-inductance and lowering the series resistance of the receiving coil. Therefore, multiple layers of metal and thick metal are more desirable, but in reality this is difficult to fabricate using conventional micromachining techniques.
Y. C. Tai, F. Jiang, Y. Xu, M. Liger, S. Ho and C. M. Ho, Flexible MEMS skins: technologies and applications. Proceedings, Pacific Rim MEMS Workshop, Xiamen, China, 2002 describes a shear-stress sensors array integrated on a flexible polymer thin film, fabricated with a parylene/metal thin film technology.